Among the conventionally-known lawn mowers is one disclosed in Japanese Patent Application Laid-open Publication H-06-17839 (hereinafter referred to as “the relevant patent literature”), where an output shaft extends downwardly of a power source and a grass cutting blade is mounted on the output shaft via a drive hub, a friction member, etc. More specifically, in the lawn mower disclosed in the relevant patent literature, the drive hub is provided on the output shaft extends downward from the power source, the friction member has a frictional surface adapted to be pressed against a frictional surface of the drive hub, and a blade holder is connected to the friction member. Further, the frictional surface of the drive hub is formed in a convex truncated cone shape, while the frictional surface of the friction member is formed in a concave truncated cone shape.
By the friction member being pressed via a coned disk spring, the frictional surface of the friction member is brought into contact with the frictional surface of the drive hub, so that driving force of the power source is transmitted to the grass cutting blade via the output shaft, the drive hub, the friction member and the blade holder. Thus, the grass cutting blade can be rotated to cut grass.
If the grass cutting blade hits an obstacle, such as a sprinkler or stone, during grass cutting operation, the frictional surface of the friction member can slide relative to the frictional surface of the drive hub and thereby absorb an impact load input to the grass cutting blade. Thus, it is possible to suppress the input impact load from being transmitted to the output shaft and thereby protect the output shaft.
However, in the lawn mower disclosed in the relevant patent literature, where the frictional surface of the drive hub is formed in a convex truncated cone shape while the frictional surface of the friction member is formed in a concave truncated cone shape, the frictional surface of the friction member cannot be slid relative to the frictional surface of the drive hub in a vertical (up-down) direction. Thus, when the grass cutting blade hits an obstacle, such as a sprinkler or stone, so that an impact load that acts on the friction member is input in such a manner as to move the friction member in the vertical direction, a bending load acts on the output shaft.
As a measure to let go the bending load acting on the output shaft, the friction member may be constructed so as to be able to slide relative to the drive hub in the vertical direction. However, if such a measure is employed, it would become difficult to position the friction member and the blade holder, and thus, it would take much time and labor to center the grass cutting blade relative to the output shaft.